1. Field of the Invention
The present invention relates generally to pellicles employed to protect a mask used in lithographic production of microelectronic circuits or other features and, more particularly, to a method and apparatus for reduction of high frequency, sound-induced vibrations in thick pellicles.
2. Description of Related Art
Pellicles are thin, optically transparent membranes that are used in photolithography to protect patterned photomask surfaces from contamination by airborne particles. The pellicles are conventionally attached to the photomask by a metal frame. Conventional pellicles are made of organic or fluorocarbon polymers. Conventional pellicles are so thin (approximately 0.5 to 2.0 {circumflex over (l)} xc2xc m) that they do not introduce any appreciable optical distortion to the light passing through them, even if the pellicle is physically distorted from an ideal flat shape.
An emerging photolithography technique is the use of radiation having a 157 nanometer (nm) wavelength to expose the desired circuit pattern from the photomask to the resist layer on the wafer substrate. Current organic polymers available for the fabrication of thin (0.5 to 2.0 {circumflex over (l)} xc2xc m) protective pellicles used at 365 nm, 248 nm, and 193 nm exposure wavelengths are either not transparent enough or not durable enough to withstand more than a few minutes of exposure without severe degradation at the 157 nm exposure wavelength. Unfortunately, no polymers have been found with sufficient radiation durability to be used as pellicles to protect photomasks from contamination at the 157 nm exposure-wavelength. For this reason, thick or hard plate pellicles have been proposed for use at this exposure wavelength. A thick plate pellicle may include a flat, polished piece of fused silica, or fluorinated fused silica, typically 100 to 1000 {circumflex over (l)} xc2xc m thick.
Unfortunately, a thick pellicle can induce many types of undesirable optical aberrations. In particular, any change of shape of the pellicle caused by gravitational sag, aerodynamic forces, or vibration will cause the pellicle-induced optical aberrations to change. Because step-and-scan exposure methods scan the mask at high speeds during exposure, a photomask may be subject to relatively high noise levels in a stepper. It has been found that even a thick pellicle will vibrate in response to the sound waves hitting it. The vibration-induced displacement of the mask can cause image distortions that will degrade the accuracy of the image placement on the wafer.
U.S. patent application Ser. No. 09/683,748 filed Feb. 11, 2002 discloses solutions to the distortions caused by gravitational sag and aerodynamic forces, but the methods taught therein are not directed to control of higher frequency distortions caused by sound. There consequently is a need for a method and apparatus for correcting sound-induced vibration of thick pellicles.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus for correcting sound-induced vibration of pellicles used to protect photomasks.
It is another object of the present invention to provide a method and apparatus for correcting sound-induced vibration of pellicles having a thickness in the range of 100-1000 {circumflex over (l)} xc2xc m.
A further object of the invention is to provide a method and apparatus for correcting sound-induced vibration of pellicles which is particularly useful for steppers using a 157 nm exposure wavelength.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention which is directed to a method of reducing sound-induced vibrations in pellicles used in lithographic production of microelectronic features comprising providing a pellicle for protecting a photomask, monitoring background sound in the vicinity, or vibration, of the pellicle, providing opposing sound waves to the background sound, and causing the opposing sound waves to strike the pellicle to substantially cancel vibrations due to the background sound or vibration.
The pellicle has opposing surfaces, so that when the background sound strikes the pellicle on one surface of the pellicle, the method may provide that opposing sound waves may strike the pellicle on the opposing surface of the pellicle. The monitoring of the background sound may comprise determining frequency, amplitude and phase of the background sound, and the opposing sound waves may be provided with substantially the same frequency and amplitude in the same phase to the background sound.
Alternatively, when the background sound strikes the pellicle on one surface of the pellicle, the method provides that opposing sound waves may strike the pellicle on the same surface of the pellicle. The monitoring of the background sound may comprise determining frequency, amplitude and phase of the background sound, and the opposing sound waves may be provided with substantially the same frequency and amplitude in opposing phase to the background sound.
Preferably, the pellicle is mounted to a photomask to create a substantially sealed interior gap between the pellicle and photomask, and the opposing sound waves are directed into the substantially sealed interior gap to strike the pellicle and substantially cancel any sound-induced vibrations of the pellicle.
The opposing sound waves may be created by a speaker coupled to a space adjacent a surface of the pellicle, or by a speaker coupled to a sealed space between the pellicle and a photomask.
The background sound may be monitored by a microphone detecting the background sound adjacent a surface of the pellicle, or by measuring vibration of a surface of the pellicle as a result of the background sound striking the pellicle surface.
In another aspect, the present invention provides an apparatus for reducing sound-induced vibrations in pellicles used in lithographic production of microelectronic features comprising a mounting structure for coupling a pellicle to a photomask, a monitor for monitoring background sound in the vicinity of the pellicle or directly monitoring sound-induced vibrations of the pellicle, and a sound generator in the vicinity of the pellicle for generating opposing sound waves to the background sound, the sound generator being adapted to cause the opposing sound waves to strike the pellicle to substantially cancel vibrations due to the background sound.
The pellicle has opposing surfaces, and the monitor may be adjacent one surface of the pellicle and the sound generator may be adjacent the opposing surface of the pellicle. The sound generator may be adapted to determine frequency, amplitude and phase of the background sound, and provide opposing sound waves with substantially the same frequency and amplitude in the same phase to the background sound.
Alternatively, the monitor and the sound generator are adjacent the same surface of the pellicle. The sound generator may be adapted to determine frequency, amplitude and phase of the background sound, and provide opposing sound waves with substantially the same frequency and amplitude in opposing phase to the background sound.
Preferably, the mounting structure is sealed to create a substantially sealed interior gap between the pellicle and photomask, and the sound generator is coupled to the mounting structure to direct opposing sound waves into the substantially sealed interior gap to strike the pellicle and substantially cancel the background sound vibrations.
The sound generator may include a speaker coupled to a space adjacent a surface of the pellicle to create opposing sound waves, or may include a speaker coupled to a sealed space between the pellicle and a photomask to create opposing sound waves.
The monitor may comprise a microphone detecting the background sound adjacent a surface of the pellicle, or a monitor adapted to measure vibration of a surface of the pellicle as a result of the background sound striking the pellicle surface.
The method and apparatus of the present invention are most useful when the background sound and the opposing sound have a frequency above 1 Hz.